1. Field of the Invention
The present invention relates to a fuel injection valve, which has a stationary core and a movable core.
2. Description of Related Art
For example, in a fuel injection valve 300 shown in FIG. 16, a magnetic attractive force, which attracts a movable core 304 to a stationary core 302, may be increased by increasing a cross sectional area of a magnetic path of the stationary core 302 and of the movable core 304 to increase an amount of magnetic flux, which passes between the stationary core 302 and the movable core 304. Furthermore, as recited in Japanese Unexamined Patent Publication No. 2002-206468, even in a case of a fuel injection valve, in which the stationary core and the movable core are surrounded by a magnetic pipe, the magnetic attractive force can be increased by increasing the cross sectional area of the magnetic path of the stationary core and of the movable core.
However, when the cross sectional area of the movable core is increased to increase the cross sectional area of the magnetic path, the weight of the movable core is disadvantageously increased. As a result, although the magnetic attractive force is increased, the valve opening response of the fuel injection valve could be disadvantageously reduced at the time of opening the fuel injection valve for injecting fuel.
In a case of a solenoid fuel injection valve, a portion of the magnetic flux generated by a coil does not pass between the stationary core and the movable core and thus does not contribute to generation of the magnetic attractive force. However, such a portion of the magnetic flux still passes in the movable core or in the stationary core. Normally, an axial overlapping length of the stationary core, which axially overlaps with the coil, is longer than that of the movable core. Thus, the non-contributing magnetic flux, which does not contribute to the generation of the magnetic attractive force, may be generated more in the stationary core in comparison to the movable core. Thus, in a case where the cross sectional area of the magnetic path of the stationary core is equal to that of the movable core, when the amount of magnetic flux generated by the coil is increased, the stationary core is magnetically saturated first before magnetic saturation of the movable core takes place.
With reference to FIG. 17, for example, inventors of the present invention have previously proposed to increase the outer diameter of the stationary core 312 beyond the outer diameter of the movable core 310 to increase the cross sectional area of the magnetic path and thereby to increase the amount of saturation magnetic flux. In this way, the amount of magnetic flux, which contributes to the generation of the magnetic attractive force, is increased, so that the magnetic attractive force is advantageously increased without increasing the weight of the movable core. Therefore, the valve opening response is improved.
In Japanese Unexamined Patent Publication No. 2002-206468, the cross sectional area of the magnetic path of the stationary core is increased by reducing the inner diameter of the stationary core while maintaining the outer diameter of the movable core equal to the outer diameter of the stationary core.
In the cases of FIG. 17 and of Japanese Unexamined Patent Publication No. 2002-206468, when the cross sectional area of the magnetic path of the stationary core is increased beyond the cross sectional area of the magnetic path of the movable core to increase the amount of saturation magnetic flux, the valve opening response can be improved without increasing the weight of the movable core. However, the remaining magnetic flux may be increased to reduce valve closing response.
In the case of FIG. 17 where the outer diameter of the stationary core 312 is made larger than the outer diameter of the movable core 310, when a magnetic member 314 is arranged radially outward of the movable core 310, the stationary core 312 is axially opposed to both the movable core 310 and magnetic member 314. In such a case, a portion of the magnetic flux, which flows between the movable core 310 and the stationary core 312 and contributes to the generation of the magnetic attractive force for attracting the movable core 310, flows between the stationary core 312 and the magnetic member 314, so that the magnetic flux, which flows between the stationary core and the movable core, is reduced. As a result, even when the cross sectional area of the large diameter portion of the stationary core is increased by increasing the outer diameter of the stationary core relative to that of the movable core, an increase of the magnetic attractive force is not sufficient.